Electrical signal separating device having isolating and matching circuitry for split passband matching

ABSTRACT

The present invention provides within three particular aspects thereof an isolating and matching device operable to enable an electrically heatable window of a motor vehicle to be used as an efficient VHF communications aerial or antenna; wherein isolating and matching circuitry is provided for Series VHF Matching, Shunt VHF Matching and Split Passband Matching of an electrically heatable window heating element to an aerial input impedance of an aerial feeder circuit of a radio frequency receiver or transmit/receive device.

This is a continuation of application Ser. No. 027,520 filed Mar. 18,1987 abandoned which is a division of Ser. No. 563,513 filed Dec. 20,1983 now U.S. Pat. No. 4,654,669.

BACKGROUND OF THE INVENTION

This invention relates to the field of electrical signal separatingdevices or isolating and matching devices which are utilized to enable aheating element means of an electrically heatable window, i.e., a motorvehicle heated rear window (defroster) unit not designed specifically tobe an antenna or aerial and essentially aperiodic and non-resonant at RFfrequencies, to be used as an efficient radio frequency transmitting orreceiving aerial, while the window unit is used simultaneously for aheating (demisting) purpose.

The electrical signal separating function of these devices is used forseparating a radio signal from the heating element of an electricallyheated window when the same is enabled to be used as a radio aerial.

Kropielnicki U.S. Pat. No. 4,086,594 discloses a device in the field ofthe present invention and provides an electrical signal separatingdevice for separating a radio signal from the resistance heating elementof an electrically heated window, and includes the usage of an isolatingor blocking circuit which comprises a bifilar coil means arranged so asto provide high impedance paths for in-phase fluctuating currents butlow resistance paths for currents flowing from a motor vehicle D.C.power supply, whereby the bifilar coil arrangement effectively blocksthe radio frequency signals while passing the current from the D.C.power supply to the heating element, such referenced device being usedto enable a heated rear window of a motor vehicle to be used as areceiving radio aerial and for heating purposes.

Another device in the field of the present invention is KropielnickiU.S. application Ser. No. 359,304, filed Mar. 18, 1982, which will issueas U.S. Pat. No. 4,422,077 on Dec. 20, 1983, the disclosure of whichprovides an improvement of the above-referenced prior Kropielnicki U.S.patent.

Such improvement relates to enabling the use of a motor vehicleelectrically heated window as a transmitting or receiving aerial at AMor VHF/FM wavelengths, and includes matching and tuning circuitry beingoperative to effectively match the impedance of the window heatingelement to an aerial feeder circuit of a transmitter so that efficientVHF transmission, from the motor vehicle electrically heated window ofsignals received at an aerial input terminal, is effected; thisdisclosed improvement further incorporating, in combination, isolatingcircuitry comprising a bifilar coil means.

The present invention provides further advantageous and improvedembodiments in this field of invention and further provides specificimprovements of the devices disclosed in the above-referenced U.S.patents, which improvements pertain, in particular, to improvedisolating and/or matching circuitry embodiments which have significantadvantages over the prior art recited hereinabove.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the present invention, novel isolating and matchingdevices are provided to enable a heating element of a motor vehicleelectrically heatable window, not designed specifically to be an antennaor aerial and essentially aperiodic and non-resonant at RF frequencies,to be used as either an efficient receiving aerial or a transmittingaerial in the VHF band.

More specifically, the present invention provides specific improvedembodiments which are defined and distinctly set forth hereinafter asVHF matching devices of a series, shunt, or split passband kind or type.

Accordingly, in a first aspect of the invention, a Series VHF Matchingembodiment is provided wherein the matching circuitry thereof includesinput circuit means, operatively connected with isolating circuitry andsaid heating element, which is operable to bring the heating element toseries resonance near the center of the VHF band; whereby, via thisdisclosed isolating and matching device, the motor vehicle electricallyheatable window forms together with such input circuit means a seriesresonance in the VHF band and becomes usable as an efficient VHFreceiving aerial. Further provided in such matching circuitry is aparallel resonant circuit means directly coupled with such input circuitmeans to form therewith a near-optimum matching circuit.

A second aspect of the invention is provided by a Shunt VHF Matchingembodiment, which includes matching circuitry having an input circuitmeans operatively connected with isolating circuitry and the heatingelement of a motor vehicle electrically heatable window and is operablewith an inductance of the isolating circuitry to bring the heatingelement to resonance near the center of the VHF band; whereby, via suchisolating and matching device, a motor vehicle electrically heatablewindow forms together with such input circuit means a parallel resonantmeans in the VHF band and becomes usable as an efficient VHF receivingaerial. Further included in such matching circuitry is a parallelresonant circuit means operable with the beforementioned input circuitmeans and isolating circuitry and heating element to form therewith aVHF broad-band matching characteristic circuit.

A third aspect of the invention provides a Split Passband Matchingembodiment, which affords an isolating and matching device havingcircuit means for providing two VHF passbands, one centered on apredetermined VHF transmit frequency and the other centered on apredetermined VHF receive frequency; whereby, via such isolating andmatching device, a motor vehicle electrically heatable window becomesusable as an efficient VHF transmitting and/or receiving aerial. In thisembodiment, isolating circuit means includes means to bring a heatingelement to parallel resonance near the center of the VHF band, andmatching circuit means is provided which includes series resonator meansto produce a series resonance near the center of the VHF band, secondseries resonator means operable to produce a second series resonancenear the center of the VHF band and circuit means for providing two VHFpassbands and including two series resonant means, one being seriesresonant at a predetermined VHF transmit frequency and the other beingseries resonant at a predetermined VHF receive frequency, such twoseries resonant means being connected in parallel with each other andconnected between the beforementioned second series resonator means andan aerial feeder circuit of a transmit/receive unit.

With respect to the above-described embodiments, a more full and clearexplanation will be had by reference to the hereinbelow describeddrawings of the invention and the following detailed description of thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified block diagram of the Series VHF Matchingembodiment;

FIG. 2 depicts a detailed schematic diagram of the Series VHF Matchingembodiment shown in FIG. 1 in block diagram form;

FIG. 3 shows a simplified block diagram of the Shunt VHF Matchingembodiment;

FIG. 4 depicts a detailed schematic diagram of the Shunt VHF Matchingembodiment shown in FIG. 3 in block diagram form; and

FIG. 5 is a block diagram of the Split Passband Matching embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring first to FIG. 1, there is illustrated therein a Series VHFMatching embodiment of the present invention.

For the sake of convenience, the operation of the device depicted inFIGS. 1 and 2 will be described hereinafter in four sections

Signal frequency input and matching circuit. A motor vehicleelectrically heated window and its heating element (not shown) areisolated from a motor vehicle D.C. power supply means, connectable atthe top of C3, at signal frequencies of interest by two bi-filar coilcircuit means L1 and L2, which may be bifilar chokes. L1 has a ferriterod core and is substantially or approximately self-resonant in the VHFrange, while L2 comprises a ferrite pot core and provides a highimpedance at frequencies in the AM band. Capacitor C3 provides furtherdecoupling of the depicted heater supply line connected at the top ofthe same. Capacitors C1 and C2 effectively couple the depicted two pathsto the terminal leads of a window heater element in parallel at signalfrequencies. Components L3, C5, together with the leakage inductance ofa tapped coil L4, are arranged to bring the electrically heatable windowand/or its heating element to series resonance near the center of theVHF band. This series resonant characteristic, together with aparallel-resonant circuit L4-C7, form a near-optimum matching circuitwhich couples the terminal leads of the heating element to the input ofa VHF amplifier circuit means designated TR2. Component C5, connected inseries between L3 and tapped coil L4, has a compromise value andoperates to effectively block AM signals from the VHF amplifier inputand, in this regard, a small value is desired to reduce the capacitanceloading of the depicted AM amplifier input, but an excessively low valuewould increase the reactance slope of the series resonance at VHF andreduce the obtainable bandwidth and performance. The impedancecharacteristic of L1 is such that VHF signals are effectively blocked,but AM signals are freely passed and are coupled to AM amplifier meansTR3 by capacitor C4.

VHF amplifier. Junction FET TR2 operates in a groundedgate circuit atsignal frequency, bypassing being provided by capacitor C8. The source Sof TR2 is directly coupled to a beforementioned input matching circuitor, as shown, in FIG. 2, to the junction of L4-C7. The drain D iscoupled to the RF output circuit line having an aerial terminal (notshown) for connection to an aerial feeder circuit of a radio receiver,by inductor L5, which resonates with the drain capacitance, togetherwith strays, near VHF band-center and provides near optimum matchingbetween the impedance of the cable feeding and connected to a radioreceiver, an exemplary impedance being 100 Ω, and the drain of FET TR2.Capacitor C9 blocks the amplifier supply voltage at the drain from theRF signal output line, the amplifier supply voltage being connected tothe circuit at the depicted +12 V point at the right-hand side of L8.

AM band amplifier. Bi-polar transistor TR3 acts as an emitter-followerstage, its base being connected to AM input coupling capacitor C4 viaresistor R1 while its output is coupled from its emitter to the RFoutput line connection to a radio receiver via blocking capacitor C10and ferrite inductor or choke L7. L7 is self-resonant in the VHF rangeand effectively prevents the low output impedance of AM amplifier TR3from loading the output of VHF amplifier TR2. The reactance of VHFoutput capacitor C9 is sufficiently high in the AM range to block AMsignals from entering the VHF amplifier drain supply. Resistor R1 servesto prevent high frequency parasitic oscillations in the AM amplifiercircuit and R1 could be replaced by two separate resistors connected inseries with each other for convenience in a printed circuit board layoutof this circuit, the values of such series connected resistors being 47Ω each.

Bias-supply circuits. In order to reduce the sensitivity of theoperating parameters of the unit depicted in FIGS. 1 and 2, to supplyvoltage variations and to FET parameter variations, the section of thiscircuit centered on zener diode D1 and transistor TR1, together withassociated resistors R5 and R3, act to provide a stable bias-supplyvoltage which is connected to the gate of TR2 and to the base of TR3,via R4, with a current source being connected to the source S of TR2.Negligible variations of the operating parameters of the depictedamplifier circuits occur within the normal range of supply voltagevariations experienced in a motor vehicle. Noise and interferencepossibly occurring with the power supply voltage, supplied at thedepicted point from a motor vehicle D.C. power supply or otherappropriate source, is rejected by filter components L8, a ferriteinductor, and C11. Zener diode D3 provides protection from possiblelarge voltage transients on the supply voltage. Further with respect tofilter components L8 and C11, the produced filtered supply voltagetherefrom is connected directly to the collector of AM amplifier TR3and, via VHF ferrite inductor or choke L6, to the drain D of TR2. Twofurther protective devices are incorporated in the circuit embodiment ofFIG. 2, namely, a neon tube element and diode D2, which are connected asshown in order to give protection in the event of static dischargeoccurring at the heater element of a motor vehicle electrically heatablewindow unit during any individual or simultaneous operation or functionthereof.

With respect to the above-described isolating circuit means of thisembodiment, the same could include a mutually coupled circuit meanswhich may comprise a toroidal or U-shaped core wound with two mutuallycoupled coils of equal but opposite polarity. Other modifications andvariations of the depicted isolating circuitry may now become apparentto those skilled in the art and, for example, a mutually coupled coilcircuit means could be substituted for bifilar component L2 in a closedmagnetic circuit form.

Accordingly, the embodiment depicted in FIGS. 1 and 2 enables a heatingelement of a motor vehicle electrically heatable window, not designedspecifically to be an antenna or aerial and essentially aperiodic andnon-resonant at RF frequencies, to become usable as an efficient VHFreceiving aerial, and to afford reception of AM signals by anappropriate receiver.

L1 and L2 are depicted as ferrite-cored bifilar coil arrangements, onehaving a ferrite rod core and the other having a ferrite pot core, asset forth above.

Referring now to FIGS. 3 and 4, a Shunt VHF Matching embodiment is showntherein. A most significant aspect of the Shunt Matching embodiment, ascompared to the Series Matching embodiment depicted in FIGS. 1 and 2, isthe modified form of the VHF matching circuitry and its connection toand with the isolating circuitry. Depending on the dimensions anddetailed arrangement of a motor vehicle electrically heatable windowunit, and on the frequency range of a desired VHF band of interest, itmay be more convenient to operate a motor vehicle electrically heatablewindow unit, used as an antenna, as a parallel resonant circuit whenincorporating it in the overall matching circuit. In this regard,bifilar inductor L1 is arranged in parallel with the window heater viaits terminal leads to achieve resonance in the center of the VHF band.

For the sake of convenience, the operation of the device depicted inFIGS. 3 and 4 will be described hereinafter in four sections.

Signal frequency input and matching circuit. Depending on the dimensionsand detailed arrangement of a motor vehicle electrically heated windowunit and its convenience in relation to the pattern of the heaterelement, i.e., the size, shape, and position of the window unit and itsheating element on a specific vehicle, it is, as stated above, moreconvenient to operate the window unit as a parallel resonant circuit. Tothis end, the inductance of bifilar coil means L1, together with thereactance of 68pF capacitor C_(a), are arranged and connected with eachother and the window heater unit to bring the same to resonance near thecenter of the VHF band. Capacitor C_(a) is a compromise in its valuedesign since too small a value will result in increased susceptanceslope and loss of bandwidth/performance in the VHF band; and, a largevalue for this capacitor will jeopardize AM performance. For the sake ofexplanation, it may be stated that the abovedescribed operation of L1and C_(a) produce a parallel resonance near VHF band center, which isformed at the heater terminal lead connections. This parallel resonanceis coupled by capacitor C5 to a second parallel resonance circuit meansL4-C7 in such a way that a broad-band matching characteristic is formed.The actual values of the beforementioned components L1, L4, C5 and C7may vary depending upon the vehicle model and/or the shape and size ofthe electrically heatable window unit.

VHF amplifier. Junction FET T2 operates in a groundedgate circuit atsignal frequency, bypassing of the same being provided by capacitor C8.The source S of T2 is directly coupled to the beforementioned componentsof the matching circuitry and, in particular, to tapped coil L4. Thedrain of T2 is coupled to the RF output circuit line, which includes anaerial terminal (not shown), by ferrite inductor L5, which resonateswith the drain capacitance, together with strays, near VHF band-centerand effectively provides near optimum matching between the impedance ofthe cable feeding a radio receiver, or the impedance of an aerial feedercircuit of the same, which could be approximately 100 Ω, and the drainof T2. Capacitor C9 blocks the amplifier supply voltage at the drainside of T2 from the RF signal output circuit line, such amplifier supplyvoltage being inputted as shown in FIG. 4 at a 12 volt connection pointto the right-side of L8.

AM band amplifier. FET T3 acts as a source follower stage, its gate Gbeing connected to input coupling capacitor C4 via resistor R1, whileits output, drain side D, is coupled from its Source to the RF outputconnector line via a blocking capacitor C10 and a ferrite inductor L7,which may be a choke. L7 is self-resonant in the VHF range andeffectively prevents the low output impedance of AM amplifier means T3from loading the output of VHF amplifier means T2. The reactance of VHFoutput capacitor C9 is sufficiently high in the low frequency AM rangeto block AM signals from entering the VHF amplifier T2 drain supply.Resistor Rl serves to prevent the occurrence of high frequency parasiticoscillations in the AM amplifier circuit of T3.

Bias-supply circuits. In order to reduce the sensitivity of theoperating parameters of the depicted embodiment to supply voltagevariations and to FET parameter variations, the section of the circuitcentered on zener diode D1 and bi-polar transistor T1, together withassociated resistors R3 and R5, act to provide a stable bias voltagesupply, which is connected to the gates of T2 and T3, with a currentsource being connected to the source S of T2, as shown in FIG. 4.Negligible variations of the operating parameters of the depictedamplifier circuits occur within the normal range of supply voltagevariations experienced in a motor vehicle. Noise and interference on themotor vehicle D.C. power supply, or an equivalent amplifier supplyvoltage means, is rejected by filter components L8 and C11. Zener diodeD3 provides protection from possible large voltage transients on theamplifier supply voltage. The filtered amplifier supply voltage isconnected, as shown, to AM amplifier means T3, and, via VHF ferriteinductor or choke L6, to the drain D of VHF amplifier T2. A furtherprotective device is incorporated, this being diode D2 to giveprotection in the event of a static discharge occurring at or within theelectrically heated window unit and its heating element, during theoperation of the same.

Accordingly, the Shunt Matching embodiment depicted in FIGS. 3 and 4provides: isolating circuitry effectively coupled in parallel withterminal leads of a heating element of a motor vehicle electricallyheatable window and also connected to a motor vehicle D.C. power supply,which circuitry is operable to permit passage of heating current fromthe power supply to the heating element while isolating or blockingpassage of RF signals from the heating element to the power supply;matching circuitry operable to effectively match the impedance of theheating element to an aerial input impedance of an aerial feeder circuitof a receiver device to thereby effect efficient VHF signal reception,such matching circuitry having an input circuit means, C_(a),operatively connected with isolating circuitry and the heating elementand operable with an inductance of the isolating circuitry to bring theheating element to resonance near the center of the VHF band, whereby,the motor vehicle electrically heatable window forms together with suchinput circuit means a parallel resonant means in the VHF band andbecomes usable as an efficient VHF receiving aerial; matching circuitryincluding a parallel resonance circuit means operable with thebeforementioned input circuit means, isolating circuitry and heatingelement, to form therewith a VHF broad-band matching characteristiccircuit; isolating circuitry including bifilar coil circuit means and,as shown, two bifilar coil arrangements, one (L1) being air-cored, andthe other (L2) having a ferrite pot core; VHF band amplifier meansconnected with the beforementioned parallel resonance circuit means; AMband amplifier means to afford reception of AM signals by a receiverdevice; and, bias-supply circuit means for both the VHF band and AM bandamplifier means.

Variations and modifications of the Shunt Matching embodiment will nowbecome apparent to those skilled in the art and, in this regard, theisolating circuitry could include a mutually coupled coil circuit meanswhich may comprise a toroidal or U-shaped core wound with two mutuallycoupled coils of equal but opposite polarity, which type of device couldbe substituted for L2, in a closed magnetic circuit.

As set forth above, both the Series and Shunt VHF matching embodimentincludes matching circuitry means which operate to effectively match theimpedance of a motor vehicle electrically heatable window heatingelement to an aerial input impedance of an aerial feeder circuit of aradio receiver, to thereby effect efficient VHF signal reception.Further in this regard, the VHF band amplifier means of each of theseembodiments also serves to accomplish this purpose in the manner inwhich the operation of the same has been described hereinabove.

It will be noted that for the sake of clarity in understanding thesimilarities and differences of the Shunt Matching and Series Matchingembodiments, functionally equivalent components of the same have beendesignated like component enumerations in FIGS. 4 and 2, respectively.

With further regard to FIG. 4, the following is provided to set forthcomponent value designations and descriptions, as designed for thedepicted embodiment:

L1 4+4 turns, 9.5 mm diameter, 1.6 mm wire.

L2 101/2+101/2 turns in 30 mm ferrite pop core, 1.4 mm wire, 1.2-1.4 mH.

L4 4+4 turns, 0.5 mm wire, 6 mm diameter.

L5 8 turns-resonant at 98 MHz in circuit.

L6 16 turns-self-resonant at 98 MHz in circuit.

L8 1 mH, 1 mA choke inductor.

The hereinabove set forth embodiments for Series Matching and ShuntMatching and, in particular, their respective matching circuitarrangements, could, in principle, be used to couple a windowheater-antenna to a VHF transmitter/receiver unit, provided that therequired overall bandwidth is not excessive. However, it should berecognized that satisfactory/efficient operation of a VHF transmittermay normally demand a better quality of match than that which may beacceptable for a receiver. Thus, depending on the Q-factor of theheater-antenna, and on the separation of the VHF transmit and receivefrequencies, it may prove difficult to achieve satisfactory andefficient performance if a matching device or circuitry having a singlepassband is employed. With further regard to the dependent Q-factor of amotor vehicle electrically heatable window/elementantenna, thehereinbefore set forth Series Matching embodiment may be used for awindow element that has a low Q-factor, i.e., below 20, and thedisclosed Shunt Matching embodiment may be used with window elementshaving a high Q-factor over the VHF band, i.e., a Q-factor above 20. Inboth cases, the embodiments have been designed for a broad-band VHFreception of approximately 88-108 MHz, and an AM broadcast band of0.15-1.6 MHz.

With respect to the possible difficulty in achievingsatisfactory/efficient performance of matching circuitry having a singlepassband, a third aspect of the invention is provided in FIG. 5 whichdepicts a Split Passband Matching embodiment.

Referring to FIG. 5, the depicted embodiment provides an isolating andmatching device having two passbands which may be designed to giveimproved performance in the neighborhood of relatively widely separatedtransmit and receive frequencies, i.e., 82 and 98 MHz, as in the presentexample. In this embodiment, an isolating circuit means L1, whichcomprises an aircored bifilar inductor device, provides a path for thewindow heater supplied power from a motor vehicle D.C. power supply ordemister power supply means, and such isolating circuit means is alsodesigned to bring the heating element of the window heater to parallelresonance near the center of the VHF band. Such bifilar arrangement mayalso be designed to bring the heating element to parallel resonance nearthe center or mean frequency of two predetermined transmit and receivefrequencies. Capacitor C1, together with lead and stray inductance,forms a series resonator means at such center frequency, as does asecond series resonator means comprising L3-C2. The effect of theparallel resonance mentioned above, acting with the latter-mentionedseries resonant characteristic, is designed to provide and produces aparallel resonant characteristic in the region of a transmit frequencyand also in the region of a receive frequency, which frequencies may bepredetermined. Circuits L4-C3 and L5-C4, which are connected in parallelwith each other and connected between the second series resonator meansL3-C2 and an aerial terminal (not shown) for connection to an aerialfeeder circuit of a transmit/receive unit, provide a series resonantcharacteristic at each of two frequencies, one for transmission and theother for reception. The transmit/receive unit may be a two-way VHFcommunications transceiver device and, with further regard to circuitsL4-C3 and L5-C4, one is designed to be series resonant at apredetermined VHF transmit frequency and the other is designed to beseries resonant at a predetermined VHF receive frequency. For example,L4-C3 may be designed to be series resonant at 82 MHz for transmit, andL5-C4 may be designed to be series resonant at 98 MHz for receive, as inthe present example. The overall characteristic of this circuitembodiment may be designed to give near-optimum matching over twopassbands, one centered on a predetermined transmit frequency and theother centered on a predetermined receive frequency.

Accordingly, the split passband matching embodiment of FIG. 5 provides:isolating circuit means operable to permit passage of heating currentfrom a power supply to a heating element of a motor vehicle electricallyheatable window while isolating or blocking passage of RF signals fromsuch heating element to the power supply, and this isolating circuitmeans is also operable in a matching circuit function to bring thewindow/heating element to parallel resonance near the center of the VHFband; an isolating and matching device having circuit means forproviding two VHF passbands, one centered on a predetermined VHFtransmit frequency and the other centered on a predetermined VHF receivefrequency, whereby, a motor vehicle electrically heatable window becomesusable as an efficient VHF transmitting and/or receiving aerial; and,matching circuit means including a first series resonator means toproduce a series resonance near the center of the VHF band, a secondseries resonator means connected with said first series resonator meansand operable to produce a second series resonance near the center of theVHF band, circuit means for providing two VHF passbands and two seriesresonant means included in the circuit means for providing two VHFpassbands, one being series resonant at a predetermined VHF transmitfrequency and the other being series resonant at a predetermined receivefrequency; and, wherein the beforementioned parallel resonance actingwith the first and/or second series resonance operates to provide aparallel resonance characteristic in the region of a predetermined VHFtransmit frequency and in the region of a predetermined VHF receivefrequency.

Summarily, the Split Passband Matching embodiment provides an isolatingand matching device which is designed to be operable to providenear-optimum matching over two VHF passbands, one for transmission ofVHF signals and the other for reception of VHF signals; and, dependingon the component and element values which arise when a scheme of thistype is applied or designed for a particular window heater-antennastructure, it may be desirable to introduce impedance transformationsand other circuit modifications.

It is, of course, to be understood that the invention and the aspectsand embodiments of the same presented hereinabove, is not intended to berestricted to the details of such embodiments and many variations andmodifications will now become apparent to those skilled in the art.Therefore, the present invention need only be limited, not by thespecific disclosure herein, but only by the appended claims.

And, as the invention may be embodied in several forms without departingfrom the spirit or essential characteristics thereof, the presentembodiments are, therefore, illustrative and not restrictive, since thescope of the invention is defined by the appended claims rather than bythe description preceding them, and all changes that fall within themetes and bounds of the claims or that form their functional as well asconjointly cooperative equivalents are, therefore, intended to beembraced by those claims.

We claim:
 1. An isolating and matching device to enable the heatingelement of a motor vehicle electrically heatable window, not designedspecifically to be an antenna or aerial and essentially aperiodic andnon-resonant at VHF frequencies, to be used as a VHF transmitting and/orreceiving aerial, said device being interconnected between terminalleads of said heating element and an aerial feeder circuit of a VHFtwo-way communications transmit/receive unit, and having an input leadfor connection to a motor vehicle D.C. power supply and an aerialterminal for connection to said aerial feeder circuit, comprising incombination: isolating circuit means coupled in parallel with saidterminal leads of said heating element and with said motor vehicle D.C.power supply to permit passage of heating current from said power supplyto the said heating element while isolating or blocking passage of RFsignals from said heating element to the said power supply, saidisolating circuit means including inductor means to bring the saidheating element to parallel resonance near the center of the VHF band,and matching circuitry means connected between the said terminal leadsof said heating element and said aerial feeder circuit, for effectivelymatching the impedance of said heating element to an aerial inputimpedance of the said aerial feeder circuit, said matching circuitrymeans including circuit means for providing two VHF passbands, onecentered on a predetermined VHF transmit frequency and the othercentered on a predetermined VHF receive frequency; whereby, via saidisolating and matching device, said motor vehicle electrically heatablewindow becomes usable as an efficient VHF transmitting and/or receivingaerial.
 2. A device according to claim 1, wherein said inductor means tobring the said heating element to parallel resonance comprises bifilarinductor means.
 3. A device according to claim 1, wherein said matchingcircuitry means includes series resonator means directly connected tosaid isolating circuit means to produce a series resonance near saidcenter of the VHF band.
 4. A device according to claim 3, wherein thesaid matching circuitry means further includes second series resonatormeans, connected in parallel with said series resonator means to producea second series resonance near the said center of the VHF band.
 5. Adevice according to claim 4, wherein the said circuit means forproviding two VHF passbands is directly connected to said second seriesresonator means and includes two series resonant means, one being seriesresonant at said predetermined VHF transmit frequency and the otherbeing series resonant at said predetermined VHF receive frequency.
 6. Adevice according to claim 5 wherein said two series resonant means areconnected in parallel with each other and connected between secondseries resonation means and said aerial feeder circuit.
 7. A deviceaccording to claim 1, wherein the said circuit means for providing twoVHF passbands includes two series resonant means, one being seriesresonant at said predetermined VHF transmit frequency and the otherbeing series resonant at said predetermined VHF receive frequency.
 8. Adevice according to claim 4, wherein the said circuit means forproviding two VHF passbands includes two series resonant means, onebeing series resonant at said predetermined VHF transmit frequency andthe other being series resonant at said predetermined VHF receivefrequency, and wherein said two series resonant means are connected inparallel with each other and connected between said second seriesresonator means and said aerial feeder circuit.